1. Field of the Invention
The present invention relates to semiconductor device and a method of manufacturing the same and, more particularly to thin film gate insulated field effect transistors suitable for use in liquid crystal displays and a method of manufacturing the same.
2. Description of the Prior Art
There have been well known in the art active liquid crystal displays which are driven by thin film transistors (TFTs). The displays of this type comprise visual panels and peripheral circuits for driving the panel. The peripheral circuit is formed by attaching a single crystalline chip containing integrated circuits on a glass substrate by tab-bonding or COG (chip on glass). The visual panel comprises a plurality of pixels each being provided with a driving TFT. The TFT is usually an N-channel FET formed within an amorphous or polycrystalline semiconductor film which is electrically coupled to a respective pixel.
The carrier mobility in amorphous semiconductors, however, is substantially low, particularly the hole mobility is of the order of 0.1 cm.sup.2 /Vsec or less. The drain dielectric strength of polycrystalline semiconductors, on the other hand, can not be sufficiently improved because of dangling bonds and impurities such as oxygen collected at grain boundaries, so that it is very difficult to realize usable P-channel TFTs having sufficient characteristics. Furthermore, such TFTs possess photosensitivities, which change Vg-ID characteristics in response to irradiation. Because of this, when used in displays accompanied by back-lights which light up the visual areas, e.g. at 2000 cd, the channel regions of the TFTs have to be covered by particular blind means.
FIG. 1 is a diagram illustrating the equivalent circuit of an exemplary liquid crystal display. The diagram shows only a 2.times.2 matrix for the sake of convenience in description whereas ordinary liquid crystal displays consist of more great numbers of pixels such as those in the form of a 640.times.480 matrix, a 1260.times.960 matrix and so on. The liquid crystal display includes a liquid crystal layer 42 disposed between a pair of glass substrates 11 and 11' as shown in FIG. 2. Numeral 54 designates a polarizing plate. The inner surface of the glass substrate 11' is coated with a ground electrode 53. The inner surface of the other substrate 11 is provided with a plurality of conductive pads each constituting one pixel of the display. Each conductive pad are formed together with an N-type FET 51 whose source is electrically connected with the corresponding pad. The drains of the FETs on a similar row in the matrix is connected with a control line of the row to which control signals are supplied from a row driver 47. The gates of the N-type FETs on a similar column is connected with a control line of the column to which control signals are supplied from a column driver 46.
In the operation of the display, the column driver 46 supplies control signals of a high level to selected columns to turn on the TFTs on the column. There are, however, undersirable cases in which the on-off action of the TFTs can not sufficiently carry out so that the output voltage of the TFT (i.e. the input to the pixel) reaches only short of a predetermined high voltage level (e.g. 5V), or the output voltage does not sufficiently fall to a predetermined low voltage (e.g. 0V). The liquid crystal is intrinsically insulating and, when the TFT is turned off, the liquid crystal voltage (VLC) becomes floating. The amount of electric charge accumulated on the liquid crystal which is equivalent to a capacitance determines the VLC. The accumulated charge, however, will leak through a channel resistance RSD of the photosensitive TFT resulting in fluctuation of the VLC. Because of this, high efficiencies can not be expected when a large number of pixels are formed within one display panel.